This invention relates to airfoils and more particularly to hybrid monolithic ceramic and ceramic matrix composite airfoils with increased impact resistance.
A turbomachine, such as an industrial gas turbine for a co-generation system or a gas turbine engine for an aircraft, includes a compressor section, a combustion section, and a turbine section. As the working medium gases travel along the flow path, the gases are compressed in the compressor section, thereby causing the temperature and pressure of the gases to rise. The hot, pressurized gases are burned with fuel in the combustion section to add energy to the gases, which expand through the turbine section and produce useful work and/or thrust.
The combustion section contains airfoils, such as vanes and blades, which direct the flow of gases as they pass therethrough, thereby ensuring the proper mixing between the fuel and gases. The airfoils are, therefore, exposed to gas temperatures ranging from about 870xc2x0 C. (1600xc2x0 F.) to 1870xc2x0 C. (3400xc2x0 F.). However, the operating temperature of the turbomachine is often limited by the airfoil""s ability to withstand such temperatures for an extended period. Improving the airfoil""s temperature capabilities would, therefore, increase the combustion section""s operating temperature, which, in turn, would improve the turbomachines overall operating efficiency.
Airfoils must not only be capable of withstanding elevated temperatures, but they must also have relatively high impact resistance. For example, foreign objects occasionally enter the turbomachine during operation. Therefore, the airfoils must be capable of withstanding the impact force caused by the foreign object. Toughness is one means of determining a material""s impact resistance. Hence, toughness becomes an important design consideration because as the toughness increases, so does the airfoil""s ability to withstand and absorb the impact of foreign objects.
One method of improving the airfoil""s temperature capability includes manufacturing the airfoil from superalloys, such as nickel based superalloys. Superalloys are not only capable of withstanding elevated temperatures but also have high toughness. Superalloys, however, typically have a relatively high density, thereby increasing the overall weight of the turbomachine. Weight reduction in aircraft design is a critical issue because a decrease in weight translates to improved fuel efficiency. Designers of turbomachines are therefore encouraged to seek alternative materials, which decrease the weight of the airfoils.
One such class of alternative materials is ceramic matrix composites, which typically has a lower density than superalloys. Although ceramic matrix composites are not typically as tough as superalloys, ceramic matrix composites are currently capable of withstanding a continuous temperature of about 1200xc2x0 C. (2200xc2x0 F.). Ceramic matrix composites, however, are more expensive than superalloys. Hence, the application of ceramic matrix composites, to date, has been limited by their inherently high fabrication cost. The shape and structure of an airfoil have also limited the use of ceramic matrix composites in fabricating such parts. In order to achieve high aerodynamic efficiency, the airfoil typically has a thin cross section and sharp radius trailing edge. Airfoils constructed of superalloys typically have a trailing edge thickness of less than about 0.04 inch. Such a thickness, however, presents difficulties when manufacturing airfoils from ceramic matrix composites because ceramic matrix composites are typically constructed from two approaches, namely a layered cloth approach and a woven approach. Specifically, airfoil cross sections of less than 0.05 inch typically do not provide a sufficient thickness for creating a balanced fiber architecture for a layered cloth approach. Moreover, woven approaches suffer from the difficulty in transitioning the fibers around the acute radius, which is typically required.
Additionally, ceramic matrix composites are susceptible to erosion, thereby further limiting their application to airfoils. Particulate matter typically becomes entrained within the working fluid of the turbine. Because most of the commonly available ceramic matrix composites have significantly lower erosion rates when compared to superalloys, ceramic matrix composite airfoils are more susceptible to erosion than airfoils constructed of superalloys. Therefore, the use of ceramic matrix composites within turbomachines is currently not an attractive alternative to the use of superalloys.
Another materials approach for increasing the airfoil""s temperature capability includes manufacturing the airfoils from monolithic ceramics. Monolithic ceramics can withstand slightly greater temperatures than ceramic matrix composites. Specifically, monolithic ceramics constructed of silicon nitride (Si3N4) can withstand higher temperatures than ceramic matrix composites, such as SiC/SiC, over an equivalent time span. Monolithic ceramics also utilize raw materials which are lower in cost than ceramic matrix composites, thereby allowing monolithic ceramics to approach the cost equivalency with superalloys. Additionally, monolithic ceramics typically passes higher erosion resistance than ceramic matrix composites and superalloys. Furthermore, monolithic ceramics are not constrained from being formed into certain shapes, such as ceramic matrix composites. However, the fracture toughness values for monolithic ceramics are typically significantly less than that for both superalloys and ceramic matrix composites. Therefore, when designing airfoils and considering characteristics, such as impact resistance and fabrication cost, both ceramic matrix composites and monolithic ceramics are independently inadequate replacements for superalloys.
What is needed is a tough, cost efficient, high temperature resistant low density airfoil.
The present invention is a hybrid airfoil comprising a temperature resistant exterior layer and a tough, high impact resistant interior layer. Encapsulating the hybrid airfoil with a temperature resistant exterior layer protects the airfoil when exposed to a high temperature environment, and supporting the hybrid airfoil with a high impact resistant interior layer, thereby improves the overall impact resistance of the airfoil. Additionally, the hybrid airfoil has a lower density and is more erosion resistant than a similar airfoil constructed of a superalloy.
In one embodiment of the present invention, the airfoil has an exterior layer, which is a monolithic ceramic, and an interior layer, which is a fiber reinforced ceramic matrix composite. The monolithic ceramic provides the airfoil with temperature resistance. The fiber reinforced ceramic matrix composite""s impact resistance is greater than the monolithic ceramic""s impact resistance, thereby increasing the airfoil""s impact resistance in comparison to that of an airfoil comprised of only a monolithic ceramic. Combining the two materials into a hybrid airfoil exploits the benefits of each material. Specifically, the monolithic ceramic exterior improves the hybrid airfoil""s temperature resistance, and the fiber reinforced ceramic matrix composite interior layer improves the hybrid airfoil""s toughness and overall impact resistance. Additionally, the raw material cost of the hybrid airfoil is less expensive than the raw material cost of the same airfoil constructed of only a ceramic matrix composite. Namely combining these two layers reduces the required amount of ceramic matrix composite raw material, which is typically more expensive than monolithic ceramic. Most importantly, the density of the hybrid airfoil is less than that of an identically shaped airfoil constructed of a superalloy. Hence, the hybrid monolithic ceramic and ceramic matrix composite airfoil is a tough, cost efficient, high temperature resistant airfoil.
The monolithic ceramic may be comprised of silicon nitride (Si3N4), silicon aluminum oxynitride (SiAlON), silicon carbide (SiC), silicon oxynitride (Si2N2O), aluminum nitride (AlN), aluminum oxide (Al2O3), hafnium oxide (HfO2), zirconia (ZrO2), siliconized silicon carbide (Sixe2x80x94SiC) or other oxides, carbides or nitrides or a combination thereof. Constructing the exterior layer with a monolithic ceramic allows the airfoil to maintain its high temperature resistant characteristics. The hybrid monolithic ceramic and ceramic matrix composite airfoil can withstand both elevated temperatures within a gas turbine as well as impact from foreign objects because supporting the monolithic ceramic with a fiber reinforced ceramic matrix composite improves the airfoil""s impact resistance.
In another embodiment of the present invention the method for affixing the fiber reinforced ceramic matrix composite to the interior of the monolithic ceramic layer includes either laminating the reinforced ceramic matrix composite to the monolithic ceramic layer, creating a chemical vapor infiltrated layer on the interior surface of the monolithic ceramic layer or forming a pre-ceramic polymer pyrolysis ceramic matrix composite on the interior surface of the monolithic ceramic layer. Regardless of which method is used to affix the fiber reinforced ceramic matrix composite to the interior of the monolithic ceramic, the reinforcement fibers within the fiber reinforced ceramic matrix composite may include fibers such as silicon carbide (SiC), aluminum oxide (Al2O3), silicon nitride (Si3N4), carbon (C), or combinations thereof. The type of material used to construct the matrix within the fiber reinforced ceramic matrix composite, however, may depend upon the method used to affix the fiber reinforced ceramic matrix composite to the interior of the monolithic ceramic.
For example, if the fiber reinforced ceramic matrix composite is laminated to the monolithic ceramic, then the matrix may include a magnesium aluminum silicate, magnesium barium aluminum silicate, lithium aluminum silicate, barium strontium aluminum silicate, or barium aluminum silicate matrix or combinations thereof. Such silicate matrices are often referred to as glass ceramic matrices or composites. If the fiber reinforced ceramic matrix composite is created by a chemical vapor infiltrated layer on the interior surface of the monolithic ceramic layer, then the matrix may include a silicon carbide (SiC), silicon nitride (Si3N4), aluminum oxide (Al2O3), silicon aluminum oxynitride (SiAlON), aluminum nitride (AlN), zirconium oxide (ZrO2), zirconium nitride (ZrN), or hafnium oxide (HfO2) matrix. If the fiber reinforced ceramic matrix composite is formed by a polymer pyrolysis ceramic matrix composite on the interior surface of the monolithic ceramic layer, then the matrix may include a silicon nitrogen carbon oxygen compound, boron nitride (BN), silicon carbide (SiC) or silicon nitride (Si3N4), or mixtures thereof.